1. Field of the Invention
The present invention relates to an exhaust gas purification apparatus for an internal combustion engine. More specifically, the present invention relates to the exhaust gas purification apparatus for an internal combustion engine wherein two different NOx purification catalysts are serially disposed in the exhaust system for improving the NOx purification efficiency and the fuel cost performance as well as for improving the durability.
2. Description of the Related Art
With progress in the leaner air-fuel ratio in recent years, such as lean burn engines and direct injection engines, it has been desired to further improve the purification performance of NOx (nitrogen oxides) in the oxidized atmosphere. For example, Unexamined Japanese Patent Publication (kokai) No. Hei 5-302508 (1993) discloses such a kind of technique.
Conventionally, a zeolite-system NOx catalysts, in which a zeolite carries a transition metal, is disposed in the exhaust system of the internal combustion engine. In addition, an NOx absorbent, which absorbs NOx when the air-fuel ratio of the in-flowing exhaust gas is lean and reduces and purifies the absorbed NOx when the oxygen concentration of the in-flowing exhaust gas decreases, is disposed on the downstream side thereof.
This conventional technique has the following structure in order to solve the inconvenience, that is, a reducer, such as HC (hydro-carbonate), needs to be replenished frequently since the NOx absorbing performance of the NOx absorbent (NOx adsorptive catalyst) saturate in a short time. A zeolite-system NOx catalysts is disposed on the upstream side so that NOx is reduced and purified by the NOx catalysts at the lean air-fuel ratio. The remaining NOx is absorbed by the NOx absorbent on a later stage to elongate the time before saturation. Further, the air-fuel ratio is periodically switched between rich or theoretical air-fuel ratio when the air-fuel ratio remains lean.
However, the adsorbing ratio of the NOx adsorptive catalyst decreases in inverse proportion to the increase in concentration of the NOx in the exhaust gas. Further, the adsorption amount has an upper limit. Accordingly, such an attempt in elongation in the saturation time as in the conventional techniques is limited. Therefore, emphasis on the fuel cost performance, in spite of degradation in the fuel cost performance due to a considerably high frequency of needs for a rich air-fuel ratio, will result with difficulty in the achievement of intended NOx purification.
A defect of the zeolite-system NOx catalyst which is used on the upstream side in the conventional technique is that the NOx purification ratio and durability under the lean air-fuel ratio, if compared with the Ir-containing (Iridium-containing) selectively reducing catalyst.
Additionally, a disadvantage of the conventional technique is that the NOx purification temperature range of the zeolite-system NOx catalysts on the upstream side does not always corresponds to that of the NOx absorbing element (NOx adsorptive catalysts) on the downstream side.
Further, the zeolite-system NOx catalyst is superior in the performance of oxidization of such as HC and CO at the theoretical air-fuel ratio in the NOx purification temperature range. Accordingly, the conventional technique has another disadvantage that it is impossible to sufficiently supply the reducer at the theoretical air-fuel ratio to the NOx absorbing element on the downstream side.
As described above, the conventional technique is unsatisfactory as for improvement in the NOx purification performance and fuel cost performance as well as durability, retaining a number of problems, such as selection of catalysts to be combined with.
Therefore, it is an object of the present invention to solve the above-described disadvantages and to provide an exhaust gas purification apparatus wherein the NOx purification performance and fuel cost performance as well as durability are improved through an optimum combination of two different catalysts at the internal combustion engine which is capable of lean operation.
According to the present invention, an exhaust gas purification apparatus for an internal combustion engine capable of lean operation comprises an Ir-containing selectively reducing catalyst and an NOx adsorptive catalyst. In the Ir-containing selectively reducing catalyst, iridium is an active species thereof and NOx is selectively reduced in a lean atmosphere by HC as a reducer is disposed in an exhaust system of said internal combustion engine. The Ir-containing selectively reducing catalyst is disposed on an upstream side of the exhaust system. The NOx adsorptive catalyst adsorbs NOx in the lean atmosphere and reduce the adsorbed NOx at a theoretical air-fuel ratio or at a rich air-fuel ratio. The NOx adsorptive catalyst is disposed on a downstream side of the exhaust system.